Transport refrigeration systems for trailers maintain a desired set point for a perishable load by continuously cycling between cooling and hot gas heating modes. A Diesel engine is normally used for the compressor prime mover, with some trailer units also having an electric stand-by motor for driving the compressor when the trailer is parked near a source of electric potential. Evaporator and condenser fans or blowers are driven by the operative prime mover, such as via pulleys and associated belts linked to the drive shaft of the operative prime mover.
The Diesel engine used for the compressor prime mover is conventionally operated at a selected one of two speeds, called high and low speed, such as 2200 RPM and 1400 RPM, respectively. A Diesel engine has ample driving power for a transport refrigeration system, and is usually operated at high speed only during the initial temperature pull down, as it is desirable to rapidly bring the temperature of the served trailer space to a predetermined temperature above the selected set point temperature, at which point the engine speed is reduced to low speed. The Diesel engine will then usually operate at low speed during the ensuing cycling back and forth between cooling and heating modes, reverting to high speed to hold set point only during cooling and heating modes associated with very high and very low ambient temperatures, respectively.
Thermal capacity, even with prior art systems which include compressor unloading, is such that with continuously operated compressors, the temperature difference between the entrance and exit air from the evaporator is relatively high, removing considerable moisture from the conditioned air via condensation on the evaporator coils. To preserve a perishable load with a goal of long shelf life, it is usually desirable to remove as little moisture from the product as possible, while holding the load temperature as close to the selected set point temperature as possible, avoiding downward temperature swings which may damage the product by freezing, as well as upward temperature swings which raise the average temperature and lessen shelf life. To lessen moisture removal, a transport refrigeration system for a trailer is operated in high speed for as short a time as possible during temperature pull down. However, dropping to low speed greatly reduces the rate of conditioned air flow produced by the evaporator blower or blowers, making it difficult to obtain uniformity of load temperature during temperature pull down, especially at the back of a long trailer.
Thus, it would be desirable, and it is an object of the present invention, to be able to rapidly and uniformly bring the temperature of a perishable load to set point, without temperature swings which may damage the product by freezing or shortening shelf life.